Tissue paper is the general name for toilet paper, paper towel, facial tissues, napkins and other soft paper products. Tissue paper is a sheet-like product manufactured continuously at speeds as high as 25 m/s. The width of the manufactured paper web in tissue paper machine can be up to 6 meter and the thickness of paper web is of the order of 50 μm. The tissue paper comprises random arrangement of wood fibres and smaller particles, such as minerals, and chemicals. Tissue products have promising future markets due to their sustainable raw material and lack of competing materials. Therefore it is expected that tissue manufacturing will continue to increase, which increases the financial significance of understanding the phenomena in tissue making process.
The most common type of tissue machine is the dry crepe machine in which the sheet is dried on one drying cylinder called a Yankee cylinder. This is because the strength of the low weight sheet is not sufficient to support sheet transfer between smaller drying cylinders. The tissue sheet is adhered to the Yankee cylinder and then detached from the surface with a blade. As a result a strong microstructure—crepe folds—is generated on the web. The detaching, known as creping, generates high softness and also compresses the web so that it can be transferred from the Yankee cylinder to the paper reel without a web break. The creping dominates the softness of tissue paper but the softness is affected significantly also by a combination of chemicals, softeners, added to the tissue sheet, the moisture content of sheet in creping process, and several factors in tissue machine operation.
Softness is one of the major properties in tissue paper. Although the tissue making process is quite well known, the final tissue product may not meet the required softness quality at the end-user. One reason for suboptimal quality control is the difficulty to measure softness of the tissue paper reliably. The tissue softness can be divided to bulk and surface softness. The bulk softness can be measured quite reliably by measuring the thickness and elasticity of sheet. However, the measurement of surface softness is not straightforward. Softness of tissue paper is usually studied with softness panel tests in which people evaluate subjectively softness of tissue paper. Furthermore, several measurement devices have been developed with the goal to correlate with rating by softness panels. However, the instrumental measurements conflict often with panel test results. This is partly because of the uncertainty of factors affecting to subjective softness feeling and partly because the current devices measure the forces, which are not in the same sensitivity scale as what the human perceives with.
One major shortcoming in current devices is that they do not detect the slight surface feeling of tissue. It has been suggested that the artificial robotic fingers studied in medical applications could be applied also in measurement of surface softness of tissue paper. However, the online application of such measurement device is not currently realistic. Pawlak et Elhammoumi [1] noticed that the high softness feeling correlates with the amount of fibres extending from the surface of tissue paper. Furthermore, an imaging based laboratory measurement device to measure the fibres from folded tissue paper was presented in the same article. However, the folding of paper requires additional devices and such measurement in on-line conditions at the paper machine would be rather impossible.